1. Field of the Invention
The present invention relates to a binding fiber for bonding of flat materials containing natural fibers with hot steam, where the natural fibers are in particular wood fibers or cellulose fibers and the flat materials are in particular paper, cardboard, nonwovens, fiber mats, or fiber boards.
2. Description of the Prior Art
From DT 19 13 246, the use of fibers described above as bonding fibers for binding of nonwovens from natural or synthetic nonwovens is known, wherein said nonwovens have a significantly lower melting point than the other fibers forming the nonwovens. The binding fibers are softened by temperature treatment and as a result are integrally bonded with the other fibers. DT 1913 246 furthermore proposes that bicomponent fibers are used as binding fibers, in particular in the core/sheath structure, wherein the core component polyamide 6 and the sheath component consists of a copolyamide from aminocaproic acid and amino undecanoic acid. In this instance, the bonding of the nonwovens is done at 130° C. and/or 135° C.
The use of bicomponent polyamide fibers, bicomponent polyester fibers, or other amorphous polyester adhesive fibers for bonding nonwovens containing natural fibers is known from DE 297 20 598 U1. These materials are believed to bond well with natural fibers with respect to polyolefins having greater polarity on a molecular basis, in particular with the cellulose molecules contained therein. In the case of bicomponent polyamide fibers, the core consists of polyamide 66 and the sheath of polyamide 6, or the core of polyamide 6 and the sheath of a copolyamide. For the bicomponent polyester fibers, bonding temperatures in the range from 100° C. to 205° C. are specified, with preferred values for the sheath higher than 140° C., particularly higher than 160° C. In the quoted example, the bonding is performed in a hot-air oven at a temperature of 182° C. It is said that bicomponent fibers can achieve a higher stability of the end product, using a crystalline sheath polymer compared to an amorphous sheath polymer. The sheath of the Unitika Fiber Melty 7080 that was selected as the example, consists of a semi-crystalline copolyester, for example. With respect to amorphous polyester bonding fibers of polyethylene terephthalate (PET) (e.g. Grilon KB of EMS-Chemie AG) it is said that they already become tacky above 70° C., due to their amorphous structure, but that this type of fiber starts crystallizing above 125° C., however, which results in achieving a PET melting point of 254° C. The bonding of such type of adhesive fiber must be performed under pressure, however.
Insulating boards from wood with densities of up to 300 kg/m3 are known from DE 10 2004 062 647 A1 and DE 10 2004 062 649 A1, which are produced with thermoplastic binding fibers, preferably in combination with a granulated synthetic resin. The binding fibers and the granulated synthetic resin are softened and therefore activated in an oven at temperatures of 130° C.-200° C., in particular 160° C. to 185° C.
DE 102 00 559 A1 describes a sound-absorbing blanket made of natural raw material fibers like wood fibers to which had been added bicomponent binding fibers from polyester of the core/sheath type, wherein the core fiber consists of a polyester with a melting point above 160° C. and the sheath consists of a polyester where the melting point is in the range between 110° C. and 130° C. The fiber mixture formed into a mat with the so-called air-laid process is molded between two hot plates, where the hot plate has a temperature of 180° C. on the top side of the mat. The molding time is selected so that the binding fibers have time to react. In addition, the mat can be stabilized in an oven with hot air at a temperature of 180° C.
DE 34 30 467 A1 describes the manufacture of man-made boards from a mat of compressed material which is constructed of wood fibers and a curable bonding agent, such as a synthetic resin binder. The curing is done in a special press by means of steam. In this case, one makes use of the circumstance that the heat transfer with steam is more effective than in air, particularly during its condensation. On the other hand, a temperature that is significantly above 100° C. cannot be achieved without substantial expenditure and equipment, due to the excess pressure of the steam compared with the ambient pressure. A slight excess pressure results in DE 34 30 467 A1 by injecting the steam in the press into the mats of compressed material in order to overcome their resistance to flow. For work at higher temperatures, superheated steam is used. But synthetic resin binders based on acrylates are available that can be cured at temperatures below 100° C.
With methods of the type known from DE 34 30 467 A1, the bonding agent, such as described in DE 102 47 413 A1, is applied in liquid form by spraying onto wood fibers, for example. During the manufacture of compressed material mats, this application represents a separate process step and moreover also requires a relatively complex system, as can also be seen from DE 102 47 413 A1. This process step could be dispensed with when using binding fibers, since binding fibers can be easily mixed among the other fibers, and since a good and uniform mixture of the other fibers is required anyway. Furthermore, mixtures from binding fibers with additional fibers can be produced homogenously and do not have the tendency to segregate again during subsequent processing. Segregation is a distinct possibility, particularly when bonding agents of the granulated type are used, such as with DE 10 2004 062 647 A1 or DE 10 2004 062 649 A1.
For this reason, there is a requirement for binding fibers that can be used with the methods of the type known from DE 34 30 467 A1 and which can be activated with hot steam at approximately ambient pressure (i.e. including an excess pressure such as that occurring and/or required for example with DE 34 30 467 A1, if necessary). The binding fibers used in the above-mentioned prior art are not suitable for this purpose, since their activation temperature is too high.
Low-melting point copolyamides are known from EP 1 153 957 B1, which in one embodiment have a melting point of smaller than 90° C. (peak maximum) when measured according to DIN 53765. The melting point temperature is adjusted in that in addition to the normally used constituents caprolactam/ω-aminocaproic acid and laurolactam/ω-aminododecanoic acid, additional ω-amino-dicarboxylic acids and additional polyamide forming agents are combined in different quantity ratios. The copolyamides known from EP 1 153 957 B1 are specifically intended and suitable for use as hot-melt adhesives for textiles. On the other hand, because of their relatively low viscosity, they are not suitable for spinning, so that binding fibers of the required type cannot be produced with them.